Add merge request filters to Search API

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Background

Several options were added to the merge request search behind feature flags but it's not yet used by the UI or Search API. This issue is to add it to the search API.

Proposal

Add the following fields to the search API:

• label_name
• include_archived
• search_level
• author_username and not_author_username
• source_branch and not_source_branch
• target_branch and not_target_branch

If the field is not available for basic search, add it to the EE Search API only.

Also update the documentation: https://docs.gitlab.com/ee/api/search.html